Three astronomers won the Nobel prize on Tuesday for
discovering that the universe is apparently being blown apart by a mysterious
force that cosmologists now call dark energy. They are Saul Perlmutter of the
Lawrence Berkeley National Laboratory in Berkeley, Calif., Brian P. Schmidt of
the Australian National University in Weston Creek, Australia, and Adam G. Riess
of the Space Telescope Science Institute and Johns Hopkins University in
Baltimore.
They were the leaders of two competing teams of astronomers who were trying to
use the exploding stars known as Type 1a supernovae as cosmic lighthouses to
measure the expansion of the universe. They were hoping to measure how fast the
universe, which has been expanding since its fiery birth in the Big Bang 14
billion years ago, was slowing down, and thus to find out if its ultimate fate
was to fall back together in what is called a Big Crunch or not. Instead, they
reported in 1998, it was inexplicably speeding up, a conclusion that nobody
would have accepted if not for the fact that both groups wound up with the same
answer.
At the time, “We were a little scared,” Dr. Schmidt said. Subsequent
cosmological measurements have confirmed that roughly 70 percent of the universe
by mass or energy consists of this antigravitational dark energy.
The most likely explanation for this bizarre behavior is a fudge factor Albert
Einstein introduced into his equations in 1917 to stabilize he universe against
collapse and then abandoned as his greatest blunder. “Every test we have made
has come out perfectly in line with Einstein’s original cosmological constant in
1917,” Dr. Schmidt said.
Quantum theory predicts that empty space should exert a repulsive force, like
dark energy, but one that is 10 to the 120th power times stronger than what the
astronomers have measured, leaving some physicists mumbling about multiple
universes.
Lawrence M. Krauss, a cosmologist at Arizona State University said, “The
discovery that the universe is dominated by the energy of empty space has
changed everything in cosmology. Nothing could, literally, not be more exciting,
because now we know nothing is almost everything!”
In the years since then the three astronomers have shared a number of awards.
Dr. Perlmutter, who led the Supernova Cosmology Project out of Berkeley, will
get half of the prize of 10 million Swedish kronor ($1.4 million). The other
half will go to Dr. Schmidt, leader of the rival High-Z Supernova Search Team,
and Dr. Riess, who was the lead author of the 1998 paper in The Astronomical
Journal, in which the dark energy result was first published. They will get
their prizes in Stockholm on Dec. 10.
A concatenation of puzzling results from an alphabet soup of satellites and
experiments has led a growing number of astronomers and physicists to suspect
that they are getting signals from a shadow universe of dark matter that makes
up a quarter of creation but has eluded direct detection until now.
Maybe.
“Nobody really knows what’s going on,” said Gordon Kane, a theorist at the
University of Michigan. Physicists caution that there could still be a
relatively simple astronomical explanation for the recent observations.
But the nature of this dark matter is one of the burning issues of science.
Identifying it would point the way to a deeper understanding of the laws of
nature and the Einsteinian dream of a unified theory of physics.
The last few weeks have seen a blizzard of papers trying to explain the
observations in terms of things like “minimal dark matter” or “exciting dark
matter,” or “hidden valley” theory, and to suggest how to look for them in
particle accelerators like the Large Hadron Collider, set to begin operation
again outside Geneva next summer.
“It could be deliriously exciting, an incredibly cool story,” said Nima
Arkani-Hamed of the Institute for Advanced Study in Princeton, N.J., who has
been churning out papers with his colleagues. “Anomalies in the sky tell you
what to look for in the collider.”
On Thursday, a team of astrophysicists working on one of the experiments
reported in the journal Nature that a cosmic ray detector onboard a balloon
flying around the South Pole had recorded an excess number of high-energy
electrons and their antimatter opposites, positrons, sailing through local
space.
The particles, they conceded, could have been created by a previously
undiscovered pulsar, the magnetized spinning remnant of a supernova explosion,
blasting nearby space with electric and magnetic fields. But, they say, a better
and more enticing explanation for the excess is that the particles are being
spit out of the fireballs created by dark matter particles colliding and
annihilating one another in space.
“We cannot disprove that the signal could come from an astrophysical object. We
also cannot eliminate a dark matter annihilation explanation based upon current
data,” said John P. Wefel of Louisiana State University, the leader of the team,
adding, “Whichever way it goes, for us it is exciting.”
The results came on the heels of a report earlier this fall from Pamela, a
satellite built by Italian, German, Russian and Swedish scientists to study
cosmic rays. Pamela scientists reported in talks and a paper posted on the
Internet that the satellite had recorded an excess of high-energy positrons.
This, they said, “may constitute the first indirect evidence of dark matter
particle annihilations,” or a nearby pulsar.
Antimatter is rare in the universe, and so looking for it is a good way of
hunting for exotic phenomena like dark matter.
Another indication that something funny is happening on the dark side of the
universe is evident in maps of the cosmic background radiation left over from
the Big Bang. Those maps, produced most recently this year by the Wilkinson
Microwave Anisotropy Probe satellite, show a haze of what seem to be charged
particles hovering around the Milky Way galaxy, according to an analysis by
Douglas Finkbeiner of the Harvard-Smithsonian Center for Astrophysics.
Adding to the mix and mystery, the European Space Agency’s Integral satellite
detected gamma rays emanating from the center of the Milky Way, suggesting the
presence of positrons there, but with much lower energies than Pamela and Dr.
Wefel’s experiments have seen.
What all this adds up to, or indeed whether it all adds up to anything at all,
depends on which observations you trust and your theoretical presumptions about
particle physics and the nature of dark matter. Moreover, efforts to calculate
the background level of high-energy particles in the galaxy are beset with messy
uncertainties. “The dark matter signal is easy to calculate,” Dr. Kane said.
“The background is much harder.”
Dark matter has teased and obsessed astronomers since the 1930s, when the
Caltech astronomer Fritz Zwicky deduced that some invisible “missing mass” was
required to supply the gravitational glue to hold clusters of galaxies together.
The idea became respectable in the 1970s when Vera C. Rubin of the Carnegie
Institution of Washington and her collaborators found from studying the motions
of stars that most galaxies seemed to be surrounded by halos of dark matter.
The stakes for dark matter go beyond cosmology. The most favored candidates for
its identity come from a theory called supersymmetry, which unifies three of the
four known forces of nature mathematically and posits the existence of a realm
of as-yet-undiscovered particles. They would be so-called wimps — weakly
interacting massive particles — which feel gravity and little else, and could
drift through the Earth like wind through a screen door. Such particles left
over from the Big Bang could form a shadow universe clumping together into dark
clouds that then attract ordinary matter.
The discovery of a supersymmetric particle would also be a boost for string
theory, the controversial “theory of everything,” and would explicate the nature
of a quarter of the universe. But until now, the dark matter particles have
mostly eluded direct detection in the laboratory, the exception being a
controversial underground experiment called Dama/Libra, for Dark Matter/Large
Sodium Iodide Bulk for Rare Processes, under the Italian Alps, where scientists
claimed in April to have seen a seasonal effect of a “dark matter wind” as the
Earth goes around its orbit.
The sky could be a different story. Dark matter particles floating in the halos
around galaxies would occasionally collide and annihilate one another in tiny
fireballs of radiation and lighter particles, theorists say.
Dr. Wefel and his colleagues have been chasing sparks in the sky since 2000,
when they flew an instrument known as ATIC, for Advanced Thin Ionization
Calorimeter, around Antarctica on a balloon at an altitude of 23 miles, looking
for high-energy particles known as cosmic rays raining from space.
In all they have made three flights, requiring them to spend the winter at the
National Science Foundation’s McMurdo Station, which Dr. Wefel described as very
pleasant. “It’s not bad until a storm moves in. You put your hand out till you
can’t see it. Then you go out and start shoveling snow,” he explained.
The Nature paper includes data from the first two balloon flights. It shows a
bump, over theoretical calculations of cosmic ray intensities, at energies of
500 billion to 800 billion electron volts, a measure of both energy and mass in
physics. One way to explain that energy bump would be by the disintegration or
annihilation of a very massive dark particle. A proton by comparison is about
one billion electron volts.
Dr. Wefel noted, however, that according to most models, a pulsar could generate
particles with even more energy, up to trillions of volts, whereas the bump in
the ATIC data seems to fall off at around 800 billion electron volts. The ATIC
results, he said, dovetail nicely with those from Pamela, which recorded a
rising number of positrons relative to electrons, but only up to energies of
about 200 billion electron volts.
Reached in China, where he was attending a workshop, Neal Weiner of New York
University, who is working with Dr. Arkani-Hamed on dark matter models, said he
was plotting ATIC data gleaned from the Web and Pamela data on the same graph to
see how they fit, which was apparently very well.
But Piergiorgio Picozza, a professor at the University of Rome and the Pamela
spokesman, said in an e-mail message that it was too soon to say the experiments
agreed. That will depend on more data now being analyzed to learn whether Pamela
continues to see more positrons as the energy rises.
Moreover, as Dr. Kane pointed out, Pamela carries a magnet that allows it to
distinguish electrons from positrons — being oppositely charged, they bend in
opposite directions going through the magnetic field. But the ATIC instrument
did not include a magnet and so cannot be sure that it was seeing any positrons
at all: no antimatter, no exotic dark matter, at least at those high energies.
But if he is right, Dr. Wefel said that the ATIC data favored something even
more exotic than supersymmetry, namely a particle that is lost in the fifth
dimension. String theory predicts that there are at least six dimensions beyond
our simple grasp, wrapped up so tightly we cannot see them or park in them. A
particle in one of these dimensions would not appear to us directly.
You could think of it as a hamster running around on a wheel in its cage. We
cannot see the hamster or the cage, but we can sort of feel the impact of the
hamster running; according to Einsteinian relativity, its momentum in the extra
dimension would register as mass in our own space-time.
Such particles are called Kaluza-Klein particles, after Theodor Kaluza and Oscar
Klein, theorists who suggested such an extra-dimensional framework in the 1920s
to unify Einstein’s general theory of relativity and electromagnetism.
Dr. Wefel’s particle would have a mass of around 620 billion electron volts.
“That’s the one that seems to fit the best,” he said in an interview. The
emergence of a sharp edge in the data, he said, “would be a smoking gun” for
such a strange particle.
But Dr. Arkani-Hamed said that Kaluza-Klein particles would not annihilate one
another at a fast enough rate to explain the strength of the ATIC signal, nor
other anomalies like the microwave haze. He and his colleagues, including Dr.
Weiner, Dr. Finkbeiner and Tracy Slatyer, also of Harvard, drawing on work by
Matthew Strassler of Rutgers, have tried to connect all the dots with a new
brand of dark matter, in which there are not only dark particles but also a
“dark force” between them.
That theory was called “a delightful castle in the sky” by Dr. Kane, who said he
was glad it kept Dr. Arkani-Hamed and his colleagues busy and diverted them from
competing with him. Dr. Kane and his colleagues favor a 200
billion-electron-volt supersymmetric particle known as a wino as the dark matter
culprit, in which case the Pamela bump would not extend to higher energies.
Dr. Wefel said he had not kept up with all the theorizing. “I’m just waiting for
one of these modelers to say here is the data, here is the model,” he said. “Fit
it out. I’m not sure I’ve seen it yet.”
Dr. Picozza said that it was the job of theorists to come up with models and
that they were proliferating.
“At the end of the story only one will be accepted from the scientific
community, but now it is too early,” he said in an e-mail message.
Sorting all this out will take time, but not forever.
Pamela is expected to come out with new results next year, and the first results
from the Fermi Gamma-ray Space Telescope, launched last summer, should also be
out soon. Not to mention the Large Hadron Collider, which will eventually smash
together protons of seven million electron volts. It is supposed to be running
next summer.
“With so many experiments, we will soon know so much more about all of this,”
Dr. Weiner said. “In a year or two, we’ll either not be talking about this idea
at all, or it will be all we’re talking about.”
An experiment that some hoped would reveal a new class of subatomic
particles, and perhaps even point to clues about why the universe exists at all,
has instead produced a first round of results that are mysteriously
inconclusive.
“It’s intellectually interesting what we got,” said Janet M. Conrad, professor
of physics at Columbia University and a spokeswoman for a collaboration that
involves 77 scientists at 17 institutions. “We have to figure out what it is.”
Dr. Conrad and William C. Louis, a physicist at Los Alamos National Laboratory,
presented their initial findings in a talk yesterday at the Fermi National
Accelerator Laboratory, outside Chicago, where the experiment is being
performed.
The goal was to confirm or refute observations made in the 1990s in a Los Alamos
experiment that observed transformations in the evanescent but bountiful
particles known as neutrinos. Neutrinos have no electrical charge and almost no
mass, but there are so many of them that they could collectively outweigh all
the stars in the universe.
Although many physicists remain skeptical about the Los Alamos findings, the new
experiment has attracted wide interest. The Fermilab auditorium was filled with
about 800 people, and talks were given at the 16 additional institutions by
other collaborating scientists. That reflected in part the hope of finding
cracks in the Standard Model, which encapsulates physicists’ current knowledge
about fundamental particles and forces.
The Standard Model has proved remarkably effective and accurate, but it cannot
answer some fundamental questions, like why the universe did not completely
annihilate itself an instant after the Big Bang.
The birth of the universe 13.7 billion years ago created equal amounts of matter
and antimatter. Since matter and antimatter annihilate each other when they come
in contact, that would have left nothing to coalesce into stars and galaxies.
There must be some imbalance in the laws of physics that led to a slight
preponderance of matter over antimatter, and that extra bit of matter formed
everything in the visible universe.
The imbalance, some physicists believe, may be hiding in the dynamics of
neutrinos.
Neutrinos come in three known types, or flavors. And they can change flavor as
they travel, a process that can occur only because of the smidgen of mass they
carry. But the neutrino transformations reported in the Los Alamos data do not
fit the three-flavor model, suggesting four flavors of neutrinos, if not more.
Other data, from experiments elsewhere, have said the additional neutrinos would
have to be “sterile” — completely oblivious to the rest of the universe except
for gravity.
The new experiment is called MiniBooNE. (BooNE, pronounced boon, is a
contraction of Booster Neutrino Experiment. “Booster” refers to a Fermilab
booster ring that accelerates protons, and “mini” was added because of plans for
a second, larger stage to the research.)
MiniBooNE sought to count the number of times one flavor of neutrino, called a
muon, turned into another flavor, an electron neutrino. The experiment slams a
beam of protons into a piece of beryllium, and the cascade of particles from the
subatomic wreckage includes muon neutrinos that fly about 1,650 feet to a
detection chamber, a tank 40 feet in diameter that contains 250,000 gallons of
mineral oil.
Most of the neutrinos fly through unscathed, but occasionally a neutrino crashes
into a carbon atom in the mineral oil. That sets off another cascade of
particles, which is detected by 1,280 light detectors mounted on the inside of
the tank.
From the pattern of the cascades, the physicists could distinguish whether the
incoming neutrino was of muon flavor or electron. To minimize the chances of
fooling themselves, they deliberately did not look at any of the electron
neutrino events until they felt they had adequately understood the much more
common muon neutrino events. They finally “opened the box” on their electron
neutrino data on March 26 and began the analysis leading to their announcement
yesterday.
For most of the neutrino energy range they looked at, they did not see any more
electron neutrinos than would be predicted by the Standard Model. That ruled out
the simplest ways of interpreting the Los Alamos neutrino data, Dr. Conrad and
Dr. Louis said.
But at the lower energies, the scientists did see more electron neutrinos than
predicted: 369, rather than the predicted 273. That may simply mean that some
calculations are off. Or it could point to a subtler interplay of particles,
known and unknown.
“It’s tantalizing,” said Boris Kayser, a Fermilab physicist not on the MiniBooNE
project. “It could be real. But this remains to be established.”
Dr. Louis said he was surprised by the results. “I was sort of expecting a clear
excess or no excess,” he said. “In a sense, we got both.”
It is the invisible material
that makes up most of the cosmos.
Now, scientists have created
the first image of dark matter
Published: 08 January 2007
The Independent
By Steve Connor, Science Editor
One of the greatest mysteries of the universe is about to be unravelled with
the first detailed, three-dimensional map of dark matter - the invisible
material that makes up most of the cosmos.
Astronomers announced yesterday that they have achieved the apparently
impossible task of creating a picture of something that has defied every attempt
to detect it since its existence was first postulated in 1933.
Scientists have known for many years that there is more to the universe than can
be seen or detected through their telescopes but it is only now that they have
been able to capture the first significant 3D-image of this otherwise invisible
material.
Unlike the ordinary matter of the planets, stars and galaxies, which can be seen
through telescopes or detected by scientific instruments, nobody has seen dark
matter or knows what it is made of, though calculations suggest that it is at
least six times bigger than the rest of the visible universe combined.
A team of 70 astronomers from Europe, America and Japan used the Hubble space
telescope to build up a picture of dark matter in a vast region of space where
some of the galaxies date back to half the age of the universe - nearly 7
billion years.
They used a phenomenon known as gravitational lensing, first predicted by Albert
Einstein, to investigate an area of the sky nine times the size of a full moon.
Gravitational lensing occurs when light from distant galaxies is bent by the
gravitational influence of any matter that it passes on its journey through
space.
The scientists were able to exploit the technique by collecting the distorted
light from half a million faraway galaxies to reconstruct some of the missing
mass of the universe which is otherwise invisible to conventional telescopes.
"We have, for the first time, mapped the large-scale distribution of dark matter
in the universe," said Richard Massey of the California Institute of Technology
in Pasadena, one of the lead scientists in the team. "Dark matter is a
mysterious and invisible form of matter, about which we know very little, yet it
dominates the mass of the universe."
One of the most important discoveries to emerge from the study is that dark
matter appears to form an invisible scaffold or skeleton around which the
visible universe has formed.
Although cosmologists have theorised that this would be the case, the findings
are dramatic proof that their calculations are correct and that, without dark
matter, the known universe that we can see would not be able to exist.
"A filamentary web of dark matter is threaded through the entire universe, and
acts as scaffolding within which the ordinary matter - including stars, galaxies
and planets - can later be built," Dr Massey said. "The most surprising aspect
of our map is how unsurprising it is. Overall, we seem to understand really well
what happens during the formation of structure and the evolution of the
universe," he said.
The three-dimensional map of dark matter was built up by taking slices through
different regions of space much like a medical CT scanner build a 3-D image of
the body by taking different X-ray "slices" in two dimensions.
Data from the Hubble telescope was supplemented by measurements from telescopes
on the ground, such as the Very Large Telescope of the European Southern
Observatory in Chile and the Japanese Subaru telescope in Hawaii.
Details of the dark matter map were released yesterday at the annual meeting of
the American Astronomical Society in Seattle and published online by the journal
Nature. The map stretches half way back to the beginning of the universe and
shows that dark matter has formed into "clumps" as it collapsed under gravity.
Other matter then grouped around these clumps to form the visible stars,
galaxies and planets.
"The 3-D information is vital to studying the evolution of the structures over
cosmic time," said Jason Rhodes of the Jet Propulsion Laboratory in Pasadena.
Astronomers have compared the task of detecting dark matter to the difficulty of
photographing a city at night from the air when only street lights are visible.
Scientists said the new images were equivalent to seeing a city, its suburbs and
country roads in daylight for the first time. Major arteries and intersections
become evident and a variety of neighbourhoods are revealed.
"Now that we have begun to map out where dark matter is, the next challenge is
to determine what it is, and specifically its relationship to normal matter," Dr
Massey said. "We have answered the first question about where the dark matter
it, but the ultimate goal will be to determine what it is."
Various experiments on Earth are under way to try to find out what dark matter
is made of. One theory is that it is composed of mysterious sub-atomic particles
that are difficult to detect because they do not interact with ordinary matter
and so cannot be picked up and identified by conventional scientific
instruments. Comparing the maps of visible matter and dark matter have already
pointed to anomalies that could prove critical to the understanding of what
constitutes dark matter.
For anyone who has been mesmerised by the sheer number of stars that make up
a clear night sky, it seems incredible that what we can see, even with a
telescope, is but a small fraction of what is actually out there. In fact, more
than 80 per cent of the material of the universe is invisible to even the best
instruments.
It is called "dark" matter because, unlike the "bright" matter of the visible
stars, galaxies and planets, it is invisible, even though its gravitational
presence can be felt. What dark matter is made of, however remains a mystery.
Fritz Zwicky, a Swiss astronomer, was the first to postulate the existence of
dark matter in 1933 when he observed clusters of galaxies beyond our own Milky
Way. Zwicky said that these distant galaxies were moving too fast to be held
together by the gravity of the visible stars they contain.
Confirmation of Zwicky's idea came in the 1970s when astronomers measured the
speed at which stars moved inside and on the outer edges of galactic discs. To
their surprise, the outer stars were travelling just as fast as the inner stars.
Gravitational theory suggested that the outermost stars should be travelling
more slowly.
The only reasonable explanation was that each galaxy had up to 10 times more
mass than could be seen. This extra material was creating the additional gravity
that kept the outer stars from slowing down.
The latest findings from the Hubble space telescope, released at the American
Astronomical Society in Seattle yesterday, suggest that dark matter forms an
invisible "scaffold" around which the ordinary matter of the stars and galaxies
have formed. The map of dark matter has been likened to a three-dimensional
X-ray of the skeleton on which the "flesh" of the visible universe is hung.
Knowing the whereabouts of the dark matter is critical to understanding how
galaxies formed and how they began to accumulate into clusters over the 13.7
billion years since the Big Bang.
Critically, comparing the distribution maps of bright and dark matter may point
to important differences between them. Several experiments on Earth are designed
to capture the elusive subatomic particles that may account for the missing
mass.
Many scientists now feel that we are on the verge of discovering what it is that
has formed such an immense part of the universe.
A strange thing happened to the universe five billion years
ago. As if God had turned on an antigravity machine, the expansion of the cosmos
speeded up, and galaxies began moving away from one another at an ever faster
pace.
Now a group of astronomers using the Hubble Space Telescope have discovered that
billions of years before this mysterious antigravity overcame cosmic gravity and
sent the galaxies scooting apart like muscle cars departing a tollbooth, it was
already present in space, affecting the evolution of the cosmos.
“We see it doing its thing, starting to fight against ordinary gravity,” Adam
Riess of the Space Telescope Science Institute said about the antigravity force,
known as dark energy. He is the leader of a team of “dark energy prospectors,”
as he calls them, who peered back nine billion years with the Hubble and were
able to discern the nascent effects of antigravity. The group reported their
observations at a news conference yesterday and in a paper to be published in
The Astrophysical Journal.
The results, Dr. Riess and others said, provide clues and place new limits on
the nature of dark energy, a mystery that has thrown physics and cosmology into
turmoil over the last decade.
“It gives us the ability to look at changes in dark energy,” he said in an
interview. “Previously, we knew nothing about that. That’s really exciting.”
The data suggest that, in fact, dark energy has changed little, if at all, over
the course of cosmic history. Though hardly conclusive, that finding lends more
support to what has become the conventional theory, that the source of cosmic
antigravity is the cosmological constant, a sort of fudge factor that Einstein
inserted into his cosmological equations in 1917 to represent a cosmic repulsion
embedded in space.
Although Einstein later abandoned the cosmological constant, calling it a
blunder, it would not go away. It is the one theorized form of dark energy that
does not change with time.
Sean Carroll, a cosmologist at the California Institute of Technology who was
not on the team, said: “Had they found the evolution was not constant, that
would have been an incredibly earthshaking discovery. They looked where no one
had been able to look before.”
The paper by Dr. Riess and his colleagues represents a sort of progress report
from the dark side, where astrophysicists have found themselves more and more as
they try to understand what is happening to the universe.
This encounter with the invisible began eight years ago, when two competing
teams of astronomers were using exploding stars known as Type 1a supernovas as
cosmic distance markers to determine the fate of the universe.
Ever since the Big Bang 14 billion years ago, the galaxies and the rest of the
universe have been flying apart like a handful of pebbles tossed in the air.
Astronomers reasoned that gravity would be slowing the expansion, and the teams
were trying to find out by how much and, thus, determine whether all would
collapse one day into a “big crunch” or expand forever.
Instead, to their surprise, the two teams, one led by Saul Perlmutter of the
University of California, Berkeley, and the other by Brian Schmidt of the Mount
Stromlo and Siding Spring Observatories in Australia, found that the universe
was speeding up instead of slowing down.
But the ground-based telescopes that the two teams used could track supernovas
to distances of just seven billion light-years, corresponding to half the age of
the universe, and the effect could have been mimicked by dust or a slight change
in the nature of the supernova explosions.
Since then, Dr. Riess, who was a member of Dr. Schmidt’s team, and his
colleagues have used the Hubble telescope to prospect for supernovas and dark
energy farther out in space or back in time.
The new results are based on observations of 23 supernovas that are more than
eight billion years in the past, before dark energy came to dominate the cosmos.
The spectra of those distant supernovas, Dr. Riess reported, appear to be
identical to those closer and more recent examples. By combining the supernova
results with data from other experiments like the NASA Wilkinson Microwave
Anisotropy Probe, Dr. Riess and his colleagues could begin to address the
evolution of dark energy.
“That’s one of the $64,000 questions,” he said. “Is dark energy changing?”
So far, he said, the results are consistent with the cosmological constant, but
other answers are also possible. The possibility that it is the cosmological
constant is a mixed blessing. Physicists concede that they do not understand it.
Dr. Carroll of Caltech said, “Dark energy makes us nervous.”
Einstein invented his constant to explain why the universe does not collapse.
After he abandoned it, the theory was resuscitated by quantum mechanics, which
showed that empty space should be bubbling with staggering amounts of repulsive
energy. The possibility that it really exists in the tiny amounts measured by
the astronomers has flummoxed physicists and string theorists.
Because it is a property of empty space, the overall force of Einstein’s
constant grows in proportion as the universe expands, until it overwhelms
everything. Other theories of dark energy like strange force fields called
quintessence or modifications to Einstein’s theory of gravity can change in more
complicated ways, rising, falling or reversing effects.
Astronomers characterize the versions of dark energy by their so-called equation
of state, the ratio of pressure to density, denoted by the letter w. For the
cosmological constant, w is minus one.
Dr. Riess and his group used their data to make the first crude measurement of
this quantity as it stood nine billion years ago. The answer, he said, was minus
one — the magic number — plus or minus about 50 percent. By comparison for more
recent times, with many more supernovas observable and thus more data, the value
is minus one with an uncertainty of about 10 percent.
“If at one point in history it’s not minus one,” Dr. Riess said, “then we have
killed the very best explanation.”
Getting to the precision needed to kill or confirm Einstein’s constant, however,
will be very difficult, he conceded. One of the biggest sources of uncertainty
is the fact that the Type 1a explosions are not completely uniform, introducing
scatter into the observations.
The Hubble is the sole telescope that can pursue supernova explosions deeply
enough to chart the early days of dark energy. The recent announcement that the
National Aeronautics and Space Administration will send astronauts to maintain
and refurbish the Hubble once again, enabling it to keep performing well into
the next decade, is a lift for Dr. Riess’s project. A new camera could extend
observations to 11 billion or 12 billion years back.
